Vertical variability of geochemical, petrophysical, and rock mechanical properties in upper Cretaceous organic-rich carbonate source rocks, Jordan

Immature organic-rich carbonate source rocks are widespread in sedimentary basins, serving as major oil shale resources and as analogs for thermally mature unconventional plays. Determining their baseline petrophysical and geomechanical properties prior to hydrocarbon generation is essential for predicting their behavior during advanced extraction. This study investigates a 20-meter vertical core from the Upper Cretaceous bituminous chalky marl interval of the Al Lajjun Graben, central Jordan, to evaluate vertical heterogeneity and identify the main controls on rock properties. An integrated workflow combined detailed core description, petrography, inorganic geochemistry, Rock-Eval pyrolysis, Multi-Sensor Core Logger (MSCL) data, spectral gamma-ray logging (SGR), and mechanical testing. Multivariate statistical methods, including principal component analysis (PCA) and hierarchical clustering on principal components (HCPC), were applied to establish a chemofacies classification. Five lithofacies, three chemofacies, and four chemostratigraphic zones were identified, indicating marked vertical variability. Bulk density ranges from 1.65 to 2.40 g/cc, porosity from 14 to 36%, and compressional wave velocity (Vp) from 2521 to 6383 m/s. Unconfined compressive strength (UCS) ranges from 19 to 132 MPa, Leeb hardness from 375 to 710 HLD, and total organic carbon (TOC) ranges from 2 to 17%. Dolomite and silica enrichment increase density and strength, whereas elevated clay content and porosity reduce mechanical competence. Porosity is negatively correlated with density, Vp, strength, and brittleness, but positively correlated with TOC. Collectively, mineralogy, diagenesis, porosity, and organic content govern vertical variability, improving reservoir characterization, exploration planning, and providing a global analog for immature unconventional carbonate source rocks.